The present invention relates to a method of creasing a product folded along a fold line, wherein a creasing nip which is formed on the fold line is moved along the fold line over the folded product in order to flatten the folded product.
A general method of this kind is known from U.S. Pat. No. 5,520,604. According to the method described therein, two creasing nips are disposed next to one another in the middle of the fold line and then moved away from one another along the fold line, each to a different end thereof.
A disadvantage of this method is that the creasing nips do not cover the entire length of the fold line because they cannot be operative between the places where the creasing nips are initially disposed. Due to the absence of a creasing action in the middle of the folded product, the latter can still gape open after the creasing operation, and this can cause problems in connection with the formation of a straight stack of folded products.
Although U.S. Pat. No. 3,931,963 discloses rolling a creasing nip over a folded product along the entire fold line, in this known method, the folded product consists of a single sheet which acts as a cover for a pack of sheets.
Accordingly, the object of the present invention is to provide a method according to the preamble without the above disadvantage and without being restricted to creasing a single folded sheet.
To this end, according to the present invention, the creasing nip is moved successively in opposite directions along the fold line over the folded product.
As a result a number of sheets double folded together is pressed flat without the sheets of the pack of sheets pressed flat on the fold line being shifted in the direction in which the creasing nip moves over the folded pack. The latter occurs if a nip is moved over a pack of sheets as explained in U.S. Pat. No. 3,008,709.
Since, according to the present invention, the creasing nip is successively moved in opposite directions over a pack of sheets, the sheet shift produced on movement in one direction is cancelled by a shift produced on movement in the opposite direction.
The present invention also relates to a device for performing the method described hereinbefore, comprising nip-forming means for forming the creasing nip on the fold line and transport means for producing a relative movement between the formed creasing nip and the folded product, wherein the transport means are provided with reversing means which, after displacement of the creasing nip along the fold line in one direction, the creasing nip is displaced in the opposite direction along the fold line.
As a result, a simple embodiment is obtained which can be disposed directly after a device in which the product is folded along a fold line by folding rollers, the folded product being held fast by the folding rollers during the creasing operation.
An advantageous embodiment of a device in which the creasing nip is formed by a pair of rollers for creasing a product folded along a fold line, comprises two fixed guide strips which extend between the nip-forming roller pair along the fold line, the folded product being adapted to be brought with its fold line between the two guide strips.
As a result, a number of double folded sheets creased on the fold line cannot readily shift in the transverse direction with respect to the fold line.
In addition, the guide strips are practically immovable in the creasing nip in a direction parallel to the direction in which the axis of the nip-forming roller extends and movable in a direction perpendicular to the flat plane traversed by the creasing nip. As a result, folded products of different thicknesses can be brought between the guide plates without the guide plates being able to shift in the creasing nip plane and thus affecting the fold quality.
In another advantageous feature of the device according to the present invention, the guide strips form an acute angle with the direction of the axis of the nip-forming roller in the direction in which the axis of the nip-forming roller extends.
As a result, a folded pack of sheets can readily be fed into the funnel formed between the guide plates and be already flattened to some extent before entering the creasing nip.
If the guide strips in a direction in which the creasing nip is displaceable has a first dimension larger than the longest fold line of a folded product for creasing and if the maximum distance over which the creasing roller is adapted to roll over the guide strip corresponds to said first dimension, a gradual transition is obtained between the creasing nip outside a product for creasing and inside the same, so that there is no adverse collision at the start of the reciprocating stroke of a creasing cycle, which collision could damage the folded product.